The present invention relates to a stainless steel tube which is used for a automobile structure member, and more specifically, to a stainless steel tube which exhibits excellent formability for secondary operation in diameter-reducing/increasing, bending, twisting and the like. In the present invention, xe2x80x9ca automobile structure memberxe2x80x9d represents members, for example, a chassis member, a bumper, a frame and the like.
A stainless steel tube used for a automobile structure member has conventionally been produced by cold forming a stainless steel sheet. In such cold forming, low-strain forming is generally carried out in order to avoid deterioration of ductility due to the forming strain.
However, in such low-strain forming, strain caused by cold forming still inevitably result in work-hardening by processing. That is, the problem of deteriorated ductility of the produced tube remains unsolved. In particular, in applications in which the product is subjected to a bending process after a diameter-reducing process, the deterioration of ductility due to the cold forming directly results in generation of cracks or too thin thickness portions in the product during the diameter-reducing process or bending process thereafter. Accordingly, a tube product produced by cold low-strain forming cannot be employed, in a satisfactory manner, for applications in which a bending process is carried out after a diameter-reducing process.
In order to solve the aforementioned problem in an advantageous manner, an object of the present invention is to provide a steel tube which is significantly more excellent in ductility than the conventional steel tube as compared at the same tensile strength level and generates no cracks and few too-thin portions therein during a diameter-reducing process or a bending process thereafter. Specifically, the object of the present invention is to provide a stainless steel tube for a automobile structure member which is excellent in the composite (diameter-reducing and bending) formability, and also excellent in formability for secondary operation in, for example, diameter-reducing/increasing, bending and twisting.
The inventors of the present invention have studied the factors for improving formability for secondary operation in, for example, diameter-reducing/increasing, bending and twisting, with respect to a stainless steel tube containing Cr. As a result, the inventors have discovered that such a stainless tube exhibits excellent formability for secondary operation only when the chemical composition, the micro structure, the tensile strength and ductility thereof are arranged in a certain range. The present invention has been achieved on the basis of this discovery.
Specifically, in the present invention, a stainless steel tube for a automobile structure member having excellent formability for secondary operation comprises: a chemical composition including not more than 0.20 mass % of C; not more than 1.5 mass % of Si; not more than 2.0 mass % of Mn; 10-18 mass % of Cr; not more than 0.03 mass % of N; Fe as the remainder and the inevitable impurities; and a micro-structure which is constituted of ferrite or ferrite and martensite, wherein the TE value defined by the following formula (1) exceeds 25,000 MPaxc2x7%.
TE value=TSxc3x97(El+21.9)xe2x80x83xe2x80x83(1) 
(In the aforementioned formula, TS represents the tensile strength in the tube axial direction (MPa), and El represents the elongation in the tube axial direction (%)). In the stainless steel tube of the present invention, the Lankford value preferably exceeds 0.5.
In the stainless steel tube of the present invention, the diameter of ferrite grain is preferably not larger than 8 xcexcm. In addition, in the stainless steel tube of the present invention, it is preferable that martensite is contained so that the area ratio thereof is not more than 30%.
Further, in the present invention, in addition to the aforementioned chemical composition, it is preferable that at least one type of element selected from the group consisting of: not more than 0.6 mass % of Cu; not more than 0.6 mass % of Ni; not more than 2.5 mass % of Mo; not more than 1.0 mass % of Nb; not more than 1.0 mass % of Ti; and not more than 1.0 mass % of V is contained.
Yet further, the present invention is directed to a automobile structure member having excellent fatigue resistance property, which member is produced by subjecting any of the aforementioned types of stainless steel tube to a secondary forming and a heat refining treatment so that the tensile strength thereof reaches not smaller than 800 MPa.